'Cosmic neutrinos are ready to take centre stage'

The search for neutrinos from deep space gives us a new vista on some of the most violent processes in the universe, says astrophysicist Ray Jayawardhana

What's so interesting about neutrinos?They are elementary particles with rather quirky properties. They hardly ever interact with matter, and that makes them really difficult to pin down. Trillions pass through your body every second but there's only maybe a 25 per cent chance that one will interact with an atom in your body in your whole lifetime.

Where do they come from?Some come from the heart of the sun; others are produced in the upper atmosphere when cosmic rays hit atoms. Then there are geoneutrinos that are produced in the Earth's interior as radioactive elements decay. The vast majority of neutrinos that pass through Earth are from those three sources. But there's a great deal of interest in detecting neutrinos that come from much farther away – cosmic neutrinos.

Why are cosmic neutrinos such a big deal?Some of the more violent phenomena in the universe produce neutrinos. So there are some really fundamental questions that cosmic neutrinos allow us to probe. So far, though, only two batches have been detected. The first were from the supernova 1987A, a star that exploded in a satellite galaxy of the Milky Way. More recently, the IceCube neutrino observatory in Antarctica reported some 28 energetic neutrinos that are almost certainly cosmic in origin.

How significant was the IceCube detection?It marks the beginnings of neutrino astronomy. Astronomy is not like other sciences; we usually don't get to put our quarry under the microscope or analyse it in the lab. We have to depend on feeble light from distant sources. By now, we've fairly well explored the electromagnetic spectrum. There are only two other potential cosmic messengers that we know of. One is gravitational waves, which still have not been detected directly. The other is cosmic neutrinos.

Do the IceCube scientists know the precise origins of the neutrinos they saw?Not yet. But the two candidate sources are the supermassive black holes at the hearts of galaxies and gamma-ray bursts, which are most likely produced by the deaths of incredibly massive stars.

What else could cosmic neutrinos reveal?There should have been neutrinos produced seconds after the big bang. With existing astronomy we can only look back to about 380,000 years after the big bang. If we could detect these "relic" neutrinos, we could look back to within seconds of the birth of the universe. The problem is that they are now low in energy, and therefore extremely difficult to detect. Present detectors are nowhere close to being sensitive enough to see them.

Can neutrinos capture the public imagination in the same way as the Higgs boson?The Higgs has been a terrific story. But neutrinos allow us to probe some really profound questions and I think that makes them truly interesting. They're ready to take centre stage.

This article appeared in print under the headline "Cosmic messengers"

Profile

Ray Jayawardhana is professor of astrophysics at the University of Toronto, Canada, and author of The Neutrino Hunters (Oneworld/Farrar, Straus and Giroux). He will be speaking at London's Royal Institution on 21 January

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"If we could detect 'relic' neutrinos, we could look back to within seconds of the birth of the universe" (Image: J. R. Bernstein)